1. Field of the Invention
The invention relates to a reflector used in a liquid crystal display device, a method of fabricating the same, and a liquid crystal display device including the same.
2. Description of the Related Art
A liquid crystal display device is grouped into a light-transmission type liquid crystal display device, a light-reflection type liquid crystal display device, and a half-transmission type liquid crystal display device in accordance with a light-source thereof.
A light-transmission type liquid crystal display device is designed to have a light-source at the rear of the device, and display images by allowing a light (called “backlight”) emitted from the light-source to pass therethrough or disallowing a light to pass therethrough.
A light-reflection type liquid crystal display device is designed to have a light-reflector at which an external light is reflected, and display images by allowing the reflected light to pass therethrough or disallowing the reflected light to pass therethrough.
A half-transmission type liquid crystal display device is designed to have a region in which the device acts as a light-transmission type liquid crystal display device, and a region in which the device acts as a light-reflection type liquid crystal display device.
Among the above-mentioned three liquid crystal display devices, a half-transmission type liquid crystal display device is often used as a display unit in a mobile device such as a mobile phone and in various electronic devices such as a digital camera both of which are expected to be used outdoors. This is because a light-transmission type liquid crystal display device used in a display unit of a note-type personal computer is accompanied with a problem that a user has difficulty in seeing images displayed in a display unit due to light reflection occurring at a surface of a display screen outdoors where sunlight is fierce.
In contrast, since a half-transmission type liquid crystal display device has a backlight source indoors, and uses an external light such as sunlight as a light source outdoors, a user can readily see displayed images indoors and outdoors. Thus, a half-transmission type liquid crystal display device is optimal as a display unit of a mobile device and a camera.
A light-reflection type liquid crystal display device is often used in a mobile game-player, for instance, since it does not need to have a light-source, and hence, has an advantage of low power consumption.
As suggested in Japanese Patent Application Publications Nos. 58-125084 and 4-243226, a reflector used in a half-transmission type or a light-reflection type liquid crystal display device is usually designed to include an organic film having a wavy surface. This is because that such a reflector has high affinity with a process (in particular, photolithography process in which a photoresist is used) of fabricating a TFT type liquid crystal display device, such a reflector can be fabricated at low costs, and such a reflector has high light-reflection characteristics superior to reflectors fabricated in accordance with other processes.
When an intensive light such as sunlight enters a liquid crystal display device including a reflector having a wavy surface, there occurs phenomenon in which, due to light interference, a surface of the liquid crystal display device is shiny, as if it emits a rainbow-color light. In a mobile device often used outdoors, such phenomenon is a critical defeat. Accordingly, there has been suggested a lot of solutions to such phenomenon.
Light interference is caused by periodicity of wavy pattern as illustrated in FIG. 1, and periodicity of an inclination angle of wavy pattern as illustrated in FIG. 2. Since light interference is dependent on a wavelength of a light, the above-mentioned phenomenon occurs.
The periodicity of wavy pattern can be reduced, for instance, by forming the wavy pattern in random. However, if the wavy pattern is formed in random, light-reflection rates in pixels would not be uniform, resulting in non-uniformity in brightness in displayed images.
Presently, a common wavy pattern is usually repeatedly formed in each of pixels, causing periodicity of wavy pattern. A common wavy pattern is formed in each of pixels for the purpose of reducing data volume of a photomask used in a photolithography process. Accordingly, if a wavy pattern is formed in random in order to avoid light interference, the data volume would be vast, resulting in much difficulty in fabrication of a photomask.
As to the periodicity of an inclination angle of wavy pattern, as long as smooth wavy pattern is formed by thermally annealing an organic film, the inclination angle is dependent on fluidity and surface tension of the organic film at a temperature dependent on a material of which the organic film is composed. Hence, it is difficult to remarkably change a profile of the inclination angle.
For instance, Japanese Patent Application Publications Nos. 2002-243923, 2002-14211, 2001-201743, 10-123508 and 11-337964 have suggested using non-periodical wavy pattern in order to reduce the above-mentioned periodicity of wavy pattern.
Japanese Patent Application Publication No. 6-27481 has suggested differentiating heights of raised and recessed portions in wavy pattern in order to reduce the periodicity of an inclination angle of wavy pattern.
Japanese Patent Application Publication No. 2002-258272 has suggested a method controlling a profile of an inclination angle of wavy pattern and a distance between a raised portion and a recessed portion in wavy patter in order to reduce the periodicity of an inclination angle of wavy pattern.
For instance, Japanese Patent Application Publication No. 2004-61767 has suggested a method of fabricating small wavy pattern by coating small light-reflective particles onto a film. Japanese Patent Application Publications Nos. 2003-114429 and 2002-357844 have suggested a method of forming small wavy pattern at a surface of a light-reflective film by controlling a temperature at which the light-reflective film is formed.
However, since the above-mentioned conventional methods of reducing light interference in a reflector include a step of forming wavy pattern at a surface of an organic film, it was not possible to delete the periodicity of an inclination angle of wavy pattern, and avoid a display screen from being colored due to light interference.
Thus, in order reduce light interference, light-diffusion adhesive is coated onto a light deflector in a half-transmission or light-reflection type liquid crystal display device to thereby avoid a display screen from being colored due to light interference. However, the light-diffusion adhesive causes reduction in a light reflection rate and reflection contrast due to light diffusion caused by the light-diffusive adhesive. In particular, in a half-transmission type liquid crystal display device, since it is not possible to coat light-diffusive adhesion only onto an area in which the liquid crystal display device acts as a light-reflection type liquid crystal display device, there is caused a problem that the light-transmission contrast is lowered to about a half.
The conventional method of forming small wavy pattern by coating small light-reflective particles onto a film, suggested in Japanese Patent Application Publication No. 2004-61767, is accompanied with the following problem.
In a half-transmission type liquid crystal display device, it is necessary to coat light-reflective particles only onto an area in which the liquid crystal display device acts as a light-reflection type liquid crystal display device, and hence, it would be necessary to pattern the light-reflective particles. However, unlike an ordinary metal film, it is quite difficult to pattern the light-reflective particles. In addition, particles often cause contaminants in a fabrication process, resulting in reduction in a fabrication yield.
The conventional method of forming small wavy pattern at a surface of a light-reflective film by controlling a temperature at which the light-reflective film is formed, suggested in Japanese Patent Application Publications Nos. 2003-114429 and 2002-357844, is accompanied with the following problem.
In these methods, small wavy pattern is formed at a surface of a light-reflective film by grain growth of the light-reflective film by controlling a temperature at which the light-reflective film (for instance, an aluminum film) is formed. A temperature at which a light-reflective film is heated for formation thereof is restricted by a maximum temperature (about 250 degrees centigrade) against which an underlying organic film has resistance. Accordingly, it is not always possible to select a desired temperature for heating the light-reflective film.
If a temperature at which a light-reflective film is formed is higher, blue lights having a short wavelength (0.4 micrometers or smaller) would be scattered by wavy pattern formed by grain growth, resulting in that the light-reflective film would be yellowed. Thus, the above-mentioned conventional methods cannot provide conditions necessary for formation of the small wavy pattern.
In addition, since common conditions are applied to a substrate in the above-mentioned conventional methods, it would not be possible to form small wavy pattern only in a part of a light-reflector.